ANDROID: vendor_hooks: Allow multiple attachments to restricted hooks

Allow up to two attachments to restricted vendor hooks to enable OEM
and vendor coexistence.

Priorities are not exposed to callers on purpose. Life's too short to
re-order the callback array with concurrent readers.

Bug: 183720636
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: I5c7aca7f69e581b4197388478d47e0da6d2893e6